This invention relates to imaging systems and, more particularly, to imaging systems using video displays.
Video displays are becoming ever more popular in medical imaging systems. For example, U.S. Pat. No. 3,894,181, issued to Mistretta, et al on July 8, 1975, U.S. Pat. No. 3,848,130, issued to Macovski on Nov. 12, 1974 and U.S. Pat. Nos. 4,204,225 and 4,204,226, issued to Mistretta on May 20, 1980, describe x-ray systems using video display systems. A number of advantages are obtained by utilization of video displays in place of conventional film displays. For example, images can be viewed in real time, without waiting for a film processing step, and video images are easier to digitize and process by computer than photographic images. In addition, archiving a video image can be done relatively inexpensively on magnetic tape or discs.
The advantages of video systems have been sufficient to cause their increasing popularity despite certain disadvantages inherent with video systems. For example, video displays have less dynamic range than photographic film and are more likely to be subjected to overload conditions caused by high signal strengths. The conventional solution for preventing system overload, or blooming, resulting from high signal strength is the incorporation of gamma correction in the video system. Gamma correction, simply stated, is a non-linear circuit response which reduces display contrast. Use of gamma correction has the advantage that high amplitude signals are relatively attenuated, and, therefore, the system is less likely to overload. However, there are significant disadvantages associated with gamma correction in certain environments. Most significantly, gamma correction normally causes some signal attenuation even at relatively low signal levels, where none is really needed. In addition, the attenuation characteristics of conventional gamma correction are frequency independent. This is a drawback in certain applications. For example, in medical imaging, frequently the high frequency information in the video signal will carry with it most of the diagnostically useful information, but the low frequency information upon which it is superimposed will display higher amplitude variations and, thus, cause overloading of video equipment which can mask the lower amplitude, but more useful, high frequency information.
As an illustration of the foregoing, consider an x-ray imaging system used to provide an image of a human patient in the chest area. One side of the patient will be uniformly irradiated with x-radiation, and the radiation passing through the patient will be detected on the other side. Typically, detection is performed by an x-ray image intensifier tube. A television camera views the optical output of an x-ray image intensifier tube. Consider the area near the heart. Each horizontal scan line in the camera will initially view a low level signal resulting from highly attenuated radiation that has passed essentially parallel to the sides of the chest wall of the patient, then view a signal resulting from radiation which has passed through the lung area of the patient and, thus, is not highly attenuated because of the substantial amount of air in the patient's lungs. The next portion of the signal will be low level due to the highly attenuated radiation that has passed through the spinal column and possibly through the heart area. Then, there is a high level signal from radiation that has passed through the lungs and, finally, a highly attenuated signal due to the chest wall. If the physician sets his video system so that he can view soft tissue in the heart area, he must use a relatively high gain setting and high x-ray intensity due to the high attenuation of x-rays in the heart area. However, the high gain and intensity cause the image in the lung area to display such a high average intensity that fine structural details in the lung area are lost because the video circuits and components, such as video cameras and CRT displays, become overloaded. This is generally manifested on the video display as blooming. Conversely, if the gain is set to a low level to view the lung area, no usable image of the heart area is formed.
Conventional gamma correction circuits address this problem by decreasing the video intensity in the lung area. However, since these gamma correction circuits are frequency insensitive, the amplitude of the useful high frequency information is severely attenuated making the images less useful diagnostically.